1. Field of the Invention
This invention relates to an electron beam deflection device and, more particularly, to a device for deflecting three electron beams emitted by an electron gum of a cathode ray tube by s magnetic field.
2. Description of the Prior Art
A cathode ray tube, used as a display device, has an electron gun and a panel at a rear portion and a forward portion thereof, respectively.
The electrons emitted from the cathode of the electron gun are accelerated by a set of electrodes of the electron gun to generate an electron beam.
If this electron beam collides against a phosphor surface applied on the panel, light is generated at a point of collision. By this emitted light, the picture information is displayed.
In a color cathode ray tube, an electron gun for generating three electron beams are used.
The three electron beams of the cathode ray tube collide against the phosphor surface on which phosphors for red, green and blue are arrayed.
The light beams of red, green and blue are produced on the respective collision points on the phosphor surface.
These light beams are mixed by an additive process and observed as colors.
In this case, the three electron beams need to be selectively struck against the phosphors for red, green and blue colors.
To this end, a color selection mechanism, having a large number of slits or rectangular or circular apertures, is arranged between the phosphor surface and the electron gun.
The picture information has two-dimensionally spreading characteristics.
For displaying a picture, the electron beams of the cathode ray tube are swept two-dimensionally.
For sweeping the electron beam, an electron beam deflection device for generating a magnetic field or an electric field is provided near the rear end of the electron gun of the cathode ray tube.
The device for deflecting the electron beam using a magnetic field is termed a deflection yoke, and includes a horizontal deflection coil, a vertical deflection coil and a core.
The perimeter of the horizontal deflection coil and the vertical deflection coil is routinely encircled by a common magnetic core.
Thus, the horizontal deflection coil, vertical deflection coil and the core are unified together.
The horizontal deflection coil and the vertical deflection coil generate mutually perpendicular magnetic fields.
If the current flowing in the respective coils is changed, the phosphor surface is two-dimensionally swept by the electron beam.
The deflection yoke is mounted on the rear side of the cathode ray tube.
Specifically, the deflection yoke is mounted, from a portion of the cathode ray tube termed a xe2x80x9cneckxe2x80x9d, for encircling a portion of the cathode ray tube termed a xe2x80x9cfunnelxe2x80x9d from outside.
This neck portion of the cathode ray tube is cylindrically-shaped and the electron gun is mounted in this cylindrical portion.
The funnel portion, consecutive to the neck portion, is spread out conically.
The inner surface of the deflection yoke is conically-shaped, in continuation to the cylindrical shape, so as to be suited to the outer surface of the neck and funnel portions.
In the above-described cathode ray tube, the electron beams proceed from the neck portion to the funnel portion.
The strength of the magnetic field, acting on the electron beams, is proportional to a reciprocal of the distance between the electron beams and the coil, in accordance with the ampere""s law.
The deflection yoke is designed to follow the outer surface of the cathode ray tube.
If the diameter of the neck portion of the cathode ray tube is reduced, it is possible to design the deflection yoke with a smaller inner diameter.
The result is the reduced distance between the electron beam and the deflection coil. The magnitude of the current necessary for deflection of the electron beam becomes smaller such that the power required for deflecting the electron beam, that is the deflection power, becomes smaller.
However, with a cathode ray tube with a smaller diameter of the neck portion, there is no alternative but to use an electron gun of a smaller diameter.
With the electron gun of the smaller diameter, the electron lens provided in its electrode portion is small in diameter. Thus, the electron lens suffers from increased aberration, thus leading to an increased spot diameter and the worsened resolution of the displayed picture.
That is, reduction in the deflection power of the deflection yoke and the improvement in the image-forming performance of an electron beam are in a trade-off relation to each other.
Up to now, for maintaining the pre-set image-forming capacity, the diameter of the neck portion cannot be reduced beyond 22 mm. That is, there is a certain limitation in reducing the diameter of the inner surface of the deflection yoke, thus presenting difficulties in reducing the deflection power.
In the color cathode ray tube, color display is by an additive process. Therefore, it is desirable that the points of collision of the three electron beams be coincident on the phosphor surface.
The points of collision of the three electron beams on the phosphor surface being brought into coincidence with one another is termed convergence. This convergence is among critical characteristics of a color cathode ray tube. In order to assure satisfactory convergence, the deflection magnetic field needs to be adjusted to high precision.
In the conventional deflection yoke, the deflection magnetic field is adjusted by adjusting the winding distribution of the deflection coil. However, the winding distribution of the deflection coil is affected by a large number of factors, such as winding position, winding density or winding tension.
For obtaining the desired deflection magnetic field, it is necessary to optimize these factors. Thus, elaborate operations are required in the designing and production of deflection yokes.
As a technique of reducing the deflection power, specifically, the square product Li2 of the inductance L of the deflection yoke and the current supplied to the deflection yoke, there is known such a technique in which the deflection yoke is enclosed in the neck portion, as disclosed in xe2x80x9cK.K.N Chang, xe2x80x9cAn Experimental In-Neck Integrated Yokexe2x80x9d, SID84, Digest, p.264.
This technique resides in having the deflection yoke enclosed in the neck portion to reduce the inner diameter of the deflection yoke to reduce the deflection power.
It is however difficult to adjust e.g. the position of the deflection coil from outside the cathode ray tube.
Also, in the publication disclosed in Y. Sano et al., xe2x80x9cA High-Deflection-Sensitivity CDT with Reef Angular Yokexe2x80x9d, SID 98 Digest, p85, there is proposed a technique in which a deflection yoke is square-shaped in keeping with the angle of deflection in the vertical and horizontal directions, the neck portion is also square-shaped in meeting therewith and a groove is formed in the core inner wall for placing the coil thereon.
In this construction, the linkage between the magnetic field generated by the deflection coil and the core becomes stronger to render it possible to reduce the deflection power.
However, with this technique, the deflection power can be reduced only by about 23%.
Also, in the above-mentioned two techniques, the deflection magnetic field is adjusted depending on the winding distribution of the deflection coil. The result is that the designing and manufacture of deflection yokes are extremely labor-consuming, as conventionally.
It is therefore an object of the present invention to provide an electron beam deflection device whereby the deflection power can be reduced and the convergence in a color cathode ray tube may be adjusted easily.
In one aspect, the present invention provides an electron beam deflection device for a cathode ray tube including a horizontal deflection yoke constituted by a first core and a first coil, and a vertical deflection yoke constituted by a second core and a second coil, wherein the horizontal deflection yoke is mounted on an electron gun side of the cathode ray tube, and wherein the vertical deflection yoke is mounted on a phosphor surface side of the cathode ray tube.
In another aspect, the present invention provides an electron beam deflection device for a cathode ray tube including a horizontal deflection yoke constituted by a first core and a first coil, and a vertical deflection yoke constituted by a second core and a second coil, wherein the first core is of a flat annular shape, with upper and lower surfaces of a through-hole thereof operating as facing magnetic poles, the horizontal deflection yoke is mounted on an electron gun side of the cathode ray tube and wherein the vertical deflection yoke is mounted on a phosphor surface side of the cathode ray tube.
The electron beam deflection device according to the present invention is constituted by a deflection yoke for deflecting the electron beam of the cathode ray tube in the horizontal direction and a deflection yoke for deflecting the electron beam of the cathode ray tube in the vertical direction.
The deflection yoke for horizontal deflection and the deflection yoke for vertical deflection are provided at respective different positions in the fore-and-aft direction.
The horizontal deflection yoke has a magnetic core having opposite magnetic poles of the flat annular shape, thereby appreciably decreasing the deflection power in the horizontal direction. The deflection magnetic field is adjusted by the cut-out on an end of the magnetic core, this adjusting the convergence of the cathode ray tube.